An alternative to keyword searching is to search by subject heading or descriptor. Collectively subject headings and descriptors are called controlled vocabulary. These controlled vocabularies are lists of words that database compilers designate as the official terms to describe certain topics.
For example: If presidential elections is an official subject heading, items about presidential candidates, electoral colleges, presidential election campaigns and popular vote might be assigned to that subject heading. A searcher looking for information about any of the above topics would have to search for presidential elections to retrieve the information.
The objective is to gather all of the information on a topic under one subject heading or descriptor.
The difference between subject headings and descriptors is largely where they are used and how they are published. Subject headings are usually used in library catalogs. The Library of Congress Subject Headings are the most widely used in the United States. Descriptors are used in article indexes. Subject Headings are published in books called lists. Descriptors are published in books called thesauri. Subject Headings can be highly structured with numerous rules about using subheadings. Descriptors are less structured but focus around a field of study (education, psychology, etc.)
Subject headings and descriptors are useful for searching by topic because they consolidate the information on a topic under one word or phrase rather than requiring the searcher to compile all of the possible keywords. In addition, subject heading lists and thesauri reveal possible broader topics (such as national elections), narrower topics (such as American presidential elections), and related topics (such as congressional elections).
For a truly comprehensive search a combination of keywords (including Boolean Operators and truncation) and controlled vocabulary need to be used. Because so many words can be used to describe the same topic, keyword searching can be inefficient and may miss a relevant item. Likewise, all of the relevant subject headings or desciptors might not be assigned to an item requiring keyword searching to retrieve it.